The technology of cold cathode discharge lamps (commonly referred to as neon lights) is relatively well known and has not changed greatly for 50 years. However, several technical challenges have prevented the widespread use of neon lights for rear lighting and signalling applications.
There are three main areas on which to concentrate when considering efficiency within a neon lighting system. The first is the cathode voltage fall, which within a normal neon lamp is approximately 150 volts for a typical electrode pair. The second is the voltage fall within the discharge, which is dependent upon the gas filling, pressure, current density and discharge length. The final area is the optical processing of the light produced, both by a reflector and a lens. The cathode voltage fall is mostly dependent upon the material used for the electrode.
It has been shown that if a discharge is constrained within a small diameter, a higher proportion of the inputted energy is used changing the state of the gas molecules, which then revert back emitting light, than with an unconstrained discharge. However, constriction raises the resistance of the discharge and thus the voltage fall. Therefore, for a given current, the heat produced within a neon light tube will be higher, a factor that is compounded by the reduced surface area. The heat generation can create surface temperature problems for the neon lamp, though its efficacy will be considerably enhanced.
Gas pressure is an important parameter when considering efficiency. In general, the lower the gas pressure, the higher the efficiency, though there is a point below which the efficiency falls again as the number of charge carriers (ions) within the lamp become insufficient to maintain a discharge. However, within the very small tubes necessary to take advantage of the gains mentioned above, the small gas volume available in a low pressure lamp may well cause a very limited life. The above action is caused by some of the gas molecules becoming adsorbed into the electrodes during operation of the lamp, thus reducing the gas pressure. This process is commonly known as gas clean-up and is an important factor in determining the life of a neon-filled discharge lamp.
The optical performance of a lamp package is also an area which promises considerable performance gains. A neon lamp emits light evenly all around it circumference, and to provide the highest efficiency, this must be gathered up and emitted in the appropriate direction for each particular application. In addition, the light must be of the correct color, and to date this has been achieved by filtering through the lens. The color of the lens is also important from the styling point of view, as people generally expect an automotive stop lamp to appear red, even when unlit.
In order to meet the technical challenges discussed above, tube diameters have been reduced, and internal diameters as small as 3 mm are being used. While this causes little problem at lower currents, the desire to design a relatively small lamp requires currents to reach up to 50 mA to achieve the required light output. This has the effect of raising the glass wall temperature up to as high as 180.degree. C. (356.degree. F.), which will melt most plastics. The temperature around the electrode will be even higher, so this is generally unacceptable.
A neon-type discharge lamp also has its life defined by three additional factors: the volume of gas present, which naturally is affected by the pressure; the operating current; and the surface area of the electrodes. Essentially, the aging process occurs as molecules of neon gas are adsorbed into the electrodes. This rate of adsorption increases rapidly with increasing current, once the capacity of the electrode is passed, and this is broadly defined by the available surface area, though it can be affected by the electrodes' configuration. Above this critical point, the electrode will start to sputter, that is, lose material into the discharge, which is then likely to be deposited onto the glass wall around the electrode. This deposition is then likely to trap more molecules of gas, thus reducing the gas pressure still further. Once this process has started, it is likely to accelerate until the gas pressure becomes so low that there are insufficient charge carriers to maintain the discharge, and the lamp will fail.